Cobra Effect · Genetics
The fly with white eyes
How one white-eyed fly showed that genes sit on chromosomes.
7 cards, read aloud in 3:15, with a test and sources.
At Columbia University, Thomas Hunt Morgan’s lab bred fruit flies by the thousand.
Fruit flies are small and cheap to keep, and a new generation grows up in about two weeks. Morgan and his students kept them in glass milk bottles, in a crowded room known as the fly room. They bred the flies generation after generation, watching for any inherited change. Almost all the flies looked alike, with dark red eyes.
In 1910 Morgan found a male fly with white eyes.
Among the red-eyed flies was a single male whose eyes were white. It was a mutation, a sudden inherited change. Morgan bred the white-eyed male with a normal red-eyed female. All of their offspring had red eyes, as Mendel’s rules would expect for a hidden, recessive feature.
In the next generation white eyes came back, but only in males.
When those red-eyed offspring bred with each other, about a quarter of the new flies had white eyes. Every one of the white-eyed flies was male. Something about being male or female was tied to the gene for eye colour. The pattern was far too regular to be chance.
The gene for eye colour sat on the chromosome that decides sex.
Female flies have two X chromosomes. Males have one X and a much smaller Y. A male gets his single X from his mother, so a white-eye gene on it has no second copy to hide it. That is why the white eyes showed up in males. For the first time, a particular gene had been tied to a particular chromosome.
Genes on the same chromosome usually travel together, but not always.
Morgan noticed that some features tended to be inherited as a group. Sometimes, though, matching chromosomes swap pieces as eggs and sperm are made, and the group splits. This swapping is called crossing over. Genes far apart on a chromosome get separated by it more often than genes close together.
A nineteen-year-old student used that idea to draw the first map of genes.
Alfred Sturtevant, an undergraduate in the fly room, realised that how often two genes were split apart could measure how far apart they were. He went home and spent most of the night working through the fly records. His map, published in 1913, set out six genes in order along the X chromosome. It showed that genes lie in a line along a chromosome, like beads on a string.
So when something odd turns up, ask what it could reveal about the ordinary.
One strange fly with white eyes led to the discovery that genes sit on chromosomes. Morgan was awarded the Nobel Prize in 1933 for that work. Fruit flies are still among the most studied animals in genetics. A single exception, followed up carefully, opened up how inheritance works.
Sources
- Thomas Hunt Morgan and the discovery of sex linkage, Scitable, Nature Education. The white-eyed fly, the crosses that followed, and what they revealed.
- The linear arrangement of six sex-linked factors in Drosophila, Alfred Sturtevant, 1913. The first map of genes, with the reasoning behind it.
- Genetic recombination and gene mapping, Scitable, Nature Education. How crossing over lets the distances between genes be measured.
Nearby ideas
- Peas that counted. How a friar counted thousands of peas and found the rules of inheritance.
- Genes are made of DNA. How dead bacteria, a purified extract and a kitchen blender showed genes are made of DNA.
- The shape that explained heredity. How an X-ray photograph and a chemical rule revealed the double helix of DNA.
- Reading the whole book of a human. How reading all three billion letters of human DNA changed what we know about ourselves.
- Scissors that find their own place. How a bacterial defence against viruses became a tool for editing genes.
- The pattern only you carry. How patterns of DNA bands reunited a family and solved a murder.